In the 4 TV series, hundreds of hours of television - the multiple times characters have visited earth and in all the previous films and TV episodes when they visited earth - not once did we see any flag except the UFP.
So, it's simply adding new material to the Trek canon that former nation states keep their old vestiges.
Uhura is from the USA (United States of Africa), Chekov is from Russia, Scotty is from Scotland, Kirk is from Iowa, Picard is from France, Riker is from Alaska. . .so even though we may not have seen flags, nations and states still exist on Earth. . . and really, there aren't that many episodes where we visited civilian places on Earth, so you can't go by what was shown in them.
OTOH, we saw a few quiet San Francisco streets in ENT and DS9, a few quiet New Orleans streets in DS9 (which looked suspiciously similar to the SF ones
), Picard's vineyard and a resteraunt in Paris in TNG and a view from Archer's hotel window in NYC in ENT. None of which were exactly the kind of places you'd have flags waving. The only place where a flag was seen was Starfleet a academy, and of course they'd fly the colours of the UFP.